Peptides are the natural ligands for many receptors that regulate critical biological functions in humans. The traditional means of gaining insight into the manner in which peptides stimulate their receptors is to evaluate how amino acid substitutions within peptides affect their biological activities (A. S. Eison, et al., Science 215, 190 (1982); R. T. Jensen, et al., Nature 309, 61 (1984)).
Typically, small numbers of variants of a peptide ("congeners") are synthesized and the relative abilities of the congeners to activate the receptor are compared. Those variants having the greatest activation activity are selected for further evaluation. (see e.g., G. Enberg, Nature 293,222 (1981); P. J. Woll, E. Rozengurt, Proc. Natl. Acad. Sci. 85, 1859 (1988)).
Recently, the development of synthetic peptide combinatorial libraries ("SPCLs") on micro beads has made available large numbers of distinct peptides for studying ligand-receptor interactions. An SPCL can include anywhere from a few to many millions of different peptides. Typically, the peptides on any one bead in an SPCL library are-the same; however, the peptide molecules can vary from-bead to bead. See for example Lam, K. S. et al., "A New Type of Synthetic Peptide Library for Identifying Ligand-Binding Activity", Nature 354:82 (1991). Thus, if it is not necessary to remove the peptides from their supports to test them in a biological system, it is straight-forward to find a bead bearing a peptide of interest.
For many situations, however, the peptides must be dissociated from the bead so that the peptide is available to interact with a test system. For example, many peptides require that their carboxyl termini be free for them to be biologically active (M. L. Moore, in Synthetic Peptides: A User's Guide, G. A. Grant, ed. (W. H. Freeman and Company, New York, 1992)); however, many peptides are attached to the beads via their carboxyl termini, thus precluding biological activity of the bound peptide. As another example, while soluble antibodies can be brought to beads, many molecules of interest cannot (e.g. G-protein coupled receptors and other membrane bound proteins such as insulin receptors). As a third example, an antibiotic peptide that kills bacteria would be ineffective or substantially less effective in an assay if the peptide could reach only those bacteria immediately contacting a particular bead.
To address this problem, the above Nature paper suggests incorporating cleavable linkers during synthesis of each bead. The problem, however, is that if the peptides are released under standard conditions, i.e., in a solution, the peptides will mix together unless a single, or at most a few beads, are assayed at one time. This approach is not practical if evaluation of many peptides is desired.